This invention relates to a display device comprising an electro-optical display medium between two supporting plates, a system of picture elements arranged in rows and columns with each picture element being constituted by two picture electrodes provided on the facing surfaces of the supporting plates, a system of row and column electrodes for driving the picture elements, the row electrodes being provided on the one supporting plate and the column electrodes being provided on the other supporting plate.
The terms row electrode and column electrode in this application may be interchanged, if desired, so that a column electrode may be meant where reference is made to a row electrode while simultaneously changing column electrode to row electrode.
A display device of this type is suitable for displaying alpha-numeric and video information with the aid of passive electro-optical display media such as liquid crystals, electrophoretic suspensions and electrochromic materials.
The known passive electro-optical display media generally have an insufficiently steep transmission characteristic with respect to the applied voltage and/or have an insufficient intrinsic memory. Owing to these properties the number of lines to be driven is limited to achieve sufficient contrast in multiplexed matrix display devices. Due to the lack of memory the information presented to a selected row via the column electrodes is to be written time and again. In addition the voltages applied to the column electrodes are not only present across the picture elements of a driven row but also across the picture elements of all other rows. Consequently picture elements receive an effective voltage during the time when they are not driven, which voltage must be sufficiently small so as not to bring a picture element to the on-state. Furthermore the ratio of the effective voltage of a picture element in the on and off-state decreases with an increasing number of row electrodes. Due to the lack of a sufficiently steep characteristic the contrast between picture elements in the on and off-states therefore decreases.
By using a switch for each picture element a memory action is obtained so that the information presented to a driven row remains present across a picture element to a sufficient extent during the time when the other row electrodes are driven, although in this case information may also get lost due to leakage currents.
A display device as described above in which diodes are used as switches is known from U.S. Pat. No. 4,223,308.
However, the use of such display devices in television systems may present problems. In a control system which is conventionally used for television such as the PAL (NTSC) system, approximately 575 (525) lines are written during each frame period of 1/25 sec (1/30 sec) distributed over an even and an odd field of approximately 288 (265) lines each per 1/50 (1/60) second. In order to obviate degradation of the liquid crystal material, display cells are preferably alternately driven with a negative and a positive voltage across the liquid crystal. For a display screen with approximately 288 (265) lines it is possible to first drive the picture cells with the information which has been presented during the odd field period and subsequently with the information which has been presented during the even field period, while the voltage across the picture cell has a different polarity during the odd field period than during the even field period. In this case interlacing does not take place, but the second picture line is written on the first picture line, the fourth on the third, and so forth. Information of the same polarity presented to a pixel is then replenished every 1/50 sec (1/60 sec) and its polarity is changed. The number of picture lines on the screen is then in fact only half the total number of lines of the two fields.
However, in order to write a complete picture of 575 (525) lines, the picture information must be presented in an interlaced manner so that the information of opposite polarity is not replenished after 1/50 (1/60) sec but after 1/25 (1/30) sec, while information of the same polarity is presented every 2/25 (1/15) sec. Since the picture cells are then driven with the same (positive or negative) voltages for a longer time, this information may get partly lost due to leakage currents. Due to inequalities between positive and negative information a flickering effect may also occur in the picture at a frequency of 25/2 (15) Hz.
In the non-prepublished Netherlands application No. 8502663, to which U.S. patent application No. 06/913,157 corresponds, filed by Applicant on Sept. 26, 1986 a display device of the type defined in the opening paragraph is described, which can be driven with the PAL (NTSC) system in which the picture quality is not reduced or is hardly reduced by flickering, while also the influence of leakage currents is considerably reduced. The said application defines with which selection and data voltages such a device can be operated and which voltages are identical in absolute value for even and odd fields. Notably the "non-selection" voltage of the odd lines in the even field, after writing a picture line of the even field, is chosen to be equal to the "non-selection" voltage after writing a picture line of the odd field. The same applies to the "non-selection" voltages of the even lines. If diodes are used as asymmetrical non-linear switching elements, driving of, for example, an LCD matrix by such a "two-level" drive may lead to such a cut-off voltage across the diodes that the leakage currents become unacceptably large.